H3C Technologies H3C S3100 Series Switches - Manual (Page 285)

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In the traditional multicast-on-demand mode, when users in different VLANs on a Layer 2 device need
multicast information, the upstream Layer 3 device must forward a separate copy of the multicast data
to each VLAN of the Layer 2 device. With the multicast VLAN or IPv6 multicast VLAN feature enabled
on the Layer 2 device, the Layer 3 multicast device sends only one copy of multicast to the multicast
VLAN or IPv6 multicast VLAN on the Layer 2 device. This avoids waste of network bandwidth and extra
burden on the Layer 3 device.
Multicast Packet Forwarding Mechanism
In a multicast model, a multicast source sends information to the host group identified by the multicast
group address in the destination address field of the IP packets. Therefore, to deliver multicast packets
to receivers located in different parts of the network, multicast routers on the forwarding path usually
need to forward multicast packets received on one incoming interface to multiple outgoing interfaces.
Compared with a unicast model, a multicast model is more complex in the following aspects.
z In the network, multicast packet transmission is based on the guidance of the multicast forwarding
table derived from the unicast routing table or the multicast routing table specially provided for
multicast.
z To process the same multicast information from different peers received on different interfaces of
the same device, every multicast packet is subject to a reverse path forwarding (RPF) check on the
incoming interface. The result of the RPF check determines whether the packet will be forwarded
or discarded. The RPF check mechanism is the basis for most multicast routing protocols to
implement multicast forwarding.
The RPF mechanism enables multicast devices to forward multicast packets correctly based on the
multicast route configuration. In addition, the RPF mechanism also helps avoid data loops caused by
various reasons.
Implementation of the RPF Mechanism
Upon receiving a multicast packet that a multicast source S sends to a multicast group G, the multicast
device first searches its multicast forwarding table:
1) If the corresponding (S, G) entry exists, and the interface on which the packet actually arrived is the
incoming interface in the multicast forwarding table, the router forwards the packet to all the
outgoing interfaces.
2) If the corresponding (S, G) entry exists, but the interface on which the packet actually arrived is not
the incoming interface in the multicast forwarding table, the multicast packet is subject to an RPF
check.
z If the result of the RPF check shows that the RPF interface is the incoming interface of the existing
(S, G) entry, this means that the (S, G) entry is correct but the packet arrived from a wrong path and
is to be discarded.
z If the result of the RPF check shows that the RPF interface is not the incoming interface of the
existing (S, G) entry, this means that the (S, G) entry is no longer valid. The router replaces the
incoming interface of the (S, G) entry with the interface on which the packet actually arrived and
forwards the packet to all the outgoing interfaces.
3) If no corresponding (S, G) entry exists in the multicast forwarding table, the packet is also subject to
an RPF check. The router creates an (S, G) entry based on the relevant routing information and